Kaganowicz, in U.S. Pat. No. 4,328,646, disclosed a method for preparing an abrasive silicon oxide coating on a substrate such as a grooved disc which includes the steps of subjecting silane (SiH.sub.4) and a gaseous, oxygen-containing compound selected from the group consisting of N.sub.2 O, H.sub.2 O, and CO.sub.2 to a glow discharge. The resulting silicon oxide (SiO.sub.x) coatings are suitable for lapping a hard material, such as diamond. Further studies of these silicon oxide coatings, as disclosed by Kaganowicz et al in U.S. Pat. No. 4,355,052, have illustrated a correlation between the refractive index and the lapping capabilities of the SiO.sub.x coating. Coatings with a refractive index between 1.1 and 1.4, for example, have provided enhanced lapping of diamond articles.
In order to use the above-mentioned methods to obtain reproducible coatings, a reasonable degree of control is needed over the "glow". It is felt that peripheral deposition, i.e., silicon oxide deposited outside the electrode area, has a detrimental effect on controlling the glow and therefore the uniformity of the coating, as well. Magnetically enhanced glow discharge systems have been employed in which magnets, behind, but electrically insulated from, the electrodes help hold the glow in the vicinity of the electrodes for a more efficient reaction of the various precursors within the glow. Even this innovation, however, has not totally eliminated all of the peripheral deposition. It is believed that a higher rate of deposition within the electrode area would further reduce peripheral deposition in magnetically enhanced glow discharge systems simply by reducing the volume of precursors which could diffuse out of the electrode area. A higher rate of deposition would also be desirable for high volume production purposes. Therefore, a method which provides increased deposition of SiO.sub.x in the electrode area and an apparatus suitable for carrying out such a method have been sought.